1. Field of the Invention
The invention relates generally to an electrophysiological (xe2x80x9cEPxe2x80x9d) system and method for providing energy to biological tissue within a biological site, and more particularly, to a radio frequency (xe2x80x9cRFxe2x80x9d) ablation system and method for determining the adequacy of contact between the system""s ablation electrodes and the tissue and the quality, e.g., depth and continuity, of any lesion resulting from RF ablation based on measurements of conduction time through the biological tissue.
2. Description of the Related Art
The heart beat in a healthy human is controlled by the sinoatrial node (xe2x80x9cS-A nodexe2x80x9d) located in the wall of the right atrium. The S-A node generates electrical signal potentials that are transmitted through pathways of conductive heart tissue in the atrium to the atrioventricular node (xe2x80x9cA-V nodexe2x80x9d) which in turn transmits the electrical signals throughout the ventricle by means of the His and Purkinje conductive tissues. Improper growth of, or damage to, the conductive tissue in the heart can interfere with the passage of regular electrical signals from the S-A and A-V nodes. Electrical signal irregularities resulting from such interference can disturb the normal rhythm of the heart and cause an abnormal rhythmic condition referred to as xe2x80x9ccardiac arrhythmia.xe2x80x9d
While there are different treatments for cardiac arrhythmia, including the application of anti-arrhythmia drugs, in many cases ablation of the damaged tissue can restore the correct operation of the heart. Such ablation can be performed percutaneously, a procedure in which a catheter is introduced into the patient through an artery or vein and directed to the atrium or ventricle of the heart to perform single or multiple diagnostic, therapeutic, and/or surgical procedures. In such case, an ablation procedure is used to destroy the tissue causing the arrhythmia in an attempt to remove the electrical signal irregularities or create a conductive tissue block to restore normal heart beat. Successful ablation of the conductive tissue at the arrhythmia initiation site usually terminates the arrhythmia or at least moderates the heart rhythm to acceptable levels. A widely accepted treatment for arrhythmia involves the application of RF energy to the conductive tissue.
In the case of atrial fibrillation (xe2x80x9cAFxe2x80x9d), a procedure published by Cox et al. and known as the xe2x80x9cMaze procedurexe2x80x9d involves the formation of continuous atrial incisions to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. While this procedure has been found to be successful, it involves an intensely invasive approach. It is more desirable to accomplish the same result as the Maze procedure by use of a less invasive approach, such as through the use of an appropriate EP catheter system providing RF ablation therapy. In this therapy, transmural ablation lesions are formed in the atria to prevent atrial reentry and to allow sinus impulses to activate the entire myocardium. In this sense transmural is meant to include lesions that pass through the atrial wall from the interior surface (endocardium) to the exterior surface (epicardium).
During ablation, to obtain a transmural lesion, it is necessary to places the electrodes on the catheter in intimate contact with the target tissue. Initial positioning of the electrodes within the atria and placement against the endocardium is typically done visually under fluoroscopy imaging. RF energy is applied to the electrodes to raise the temperature of the target tissue to a non-viable state. In general, the temperature boundary between viable and non-viable tissue is approximately 48xc2x0 Centigrade. Tissue heated to a temperature above 48xc2x0 C. becomes non-viable and defines the ablation volume. The objective is to elevate the tissue temperature, which is generally at 37xc2x0 C., fairly uniformly to an ablation temperature above 48xc2x0 C., while keeping both the temperature at the tissue surface and the temperature of the electrode below 100xc2x0 C. In clinical applications, the target temperature is set below 70xc2x0 C. to avoid coagulum formation. Coagulum formation can also be avoided by monitoring electrode impedance which is the ratio of voltage over current. A rise in electrode impedance during ablation would indicate coagulative adhesion of tissue components or tissue charring at the electrode tissue interface.
Once ablation therapy is completed, pre-ablation and post-ablation electrocardiograms (ECG) may be compared to assess lesion quality. However, because ECG amplitudes may not be highly sensitive or specific to lesion formation the accuracy of lesion quality assessment in this manner is questionable. Lesion quality assessment in this manner is also inconvenient in that it requires the use of additional ECG instrumentation.
Hence, those skilled in the art have recognized a need for an RF ablation system and method for assessing the adequacy of the contact between biological tissue and ablation electrodes and for assessing the adequacy of a lesion resulting from the application of RF energy through the electrodes. The need for providing such assessments without reliance on electrocardiogram measurements has also been recognized. The invention fulfills these needs and others.
Briefly, and in general terms, the invention is directed to methods and systems for assessing the adequacy of a lesion resulting from the application of RF energy through the electrodes and for assessing the adequacy of the contact between biological tissue and ablation electrodes.
In a first aspect, the invention relates to a method of assessing the efficacy of a biological tissue lesion between a pair of the electrodes in an ablation procedure using a catheter system having a catheter with a plurality of electrodes and a generator for providing energy to the electrodes. During a first time period, a first pulse of energy is applied to a first electrode. A first conduction time based on the time it takes for the pulse to conduct through the tissue to a second electrode is determined. During a time period subsequent to the first time, a subsequent pulse of energy is applied to the first electrode and a subsequent conduction time is determined based on the time it takes for the subsequent pulse to conduct through the tissue to the second electrode. Changes in the conduction times are monitored to assess lesion efficacy.
In another aspect, the invention relates to a system for assessing the efficacy of a biological tissue lesion between a first electrode and a second electrode positioned proximal to the biological tissue. The system includes a generator adapted to output a pulse of energy and a processor adapted to control the generator such that an energy pulse is provided to the first electrode at a first pulse-application time. The processor monitors electrical activity at the second electrode for an indication that the energy pulse has been sensed by the second electrode and records the time at which the pulse was sensed. The processor further determines a conduction time based on the difference between the pulse-application time and the pulse-sense time. The processor repeats the conduction time measurement at least once, at a time subsequent to the first pulse application time, and monitors changes in the conduction times to assess lesion efficacy.
In another aspect, the invention relates to a method of assessing the efficacy of a biological tissue lesion between pairs of electrodes in an ablation procedure using a catheter system having a catheter with a plurality of electrodes and a generator for providing a sequence of energy pulses to the electrodes. During a first time period, an initial sequence of energy pulses is applied to the plurality of electrodes such that a first pulse is applied to a first electrode, a second pulse is applied to a second electrode and so on. For each pulse, the time it takes for the pulse to conduct through the tissue to at least one of the other electrodes is determined. During a time period subsequent to the first time period, a subsequent sequence of energy pulses is applied to the plurality of electrodes in a manner similar to the initial sequence. For each pulse, the time it takes for the pulse to conduct through the tissue to at least one of the other electrodes is determined. For a selected pair of electrodes, changes in the conduction times are monitored to assess lesion efficacy.
In another aspect, the invention relates to a method of assessing the efficacy of a biological tissue lesion using a catheter system having a catheter with at least three electrodes and a generator for providing energy to the electrodes. The method includes positioning the catheter such that two electrodes spaced a distance apart are on opposite sides of the lesion and two electrodes spaced substantially the same distance apart have nonablated tissue there between. The method also includes determining the time it takes for a pulse of energy to conduct between the two electrodes on opposite sides of the lesion and determining the time it takes for a pulse of energy to conduct between the two electrodes having nonablated tissue there between. Further included in the method are calculating a difference between the conduction times and comparing the difference to a preset value. If the difference is below the preset value an indication of an adequate lesion is provided.
In another aspect, the invention relates to a method of assessing contact between the plurality of electrodes and the tissue in an ablation procedure using a catheter system having a catheter with a plurality of electrodes and a generator for providing energy to the electrodes. The method includes positioning the catheter adjacent the biological tissue such that at least one of the electrodes contacts the tissue, applying a pulse of energy to the one electrode in contact with the tissue and monitoring the electrical activity at each of the remaining electrodes for receipt of the pulse.
These and other aspects and advantages of the invention will become apparent from the following detailed description and the accompanying drawings which illustrate by way of example the features of the invention.